Combination light-emitting diode (LED) and silicon photo detector components are commonly used as sensor units. A commercially available component places the LED and detector side by side, both facing in the same direction from a common active surface with the LED. When pulsed, the LED emits a narrow cone of light in one direction and the adjacent detector generates current when illuminated from the same direction. The unit is commonly used to sense the presence of a reflecting surface over the LED, by reflecting some of the intercepted radiation back toward the detector.
There are applications employing a large number of such combination units, which applications require selective activation by a controller of one or more units concurrently. Moreover, it is sometimes desirable to align the multiple units near or adjacent one another to form an array, and a problem then arises of minimizing the effect of scattered reflected radiation on detectors remote from the emitting LED. When large numbers of such combination units are present in a system, the number of connections from the controller to the units may become excessive.
A circuit has been developed for driving or operating a multiplicity of such units which reduces the number of connections from a driving or operating source to the units and which minimizes or reduces the effects of scattered light. The circuit is part of a system comprising multiple LED-detector units operated by an enabling micro controller with a relatively low number of connections. The micro controller provides for selection of one or more LEDs for activation and selection of one or more detectors for reading its output current, for use as multiple sensors in a closely arranged array in or scattered throughout a system.
Each unit typically has four leads, one each for the LED and the detector, respectively, to provide power or source current, and two of which are used for respective connection to a sink. Each LED is activated when its source and sink lead are activated, and each detector conducts current in its detector lead when its source and sink leads are activated, and its sink detector is connected to complete the circuit.
In a known system such as that described in U.S. Pat. No. 4,886,976, entitled "Driver Circuitry for Multiple Sensors", commonly assigned, the units are electrically arranged in a matrix in which each row of the matrix corresponds to a bank of the units, and each column of the matrix corresponds to units whose detector sink leads are connected in common. In this arrangement, each unit of a bank is alternately connected to one of two detector sink busses, and each LED sink lead is connected to one of four LED sink busses. If more than four are present, then the pattern repeats. All of the units in a column have their LED source leads connected in common to a source bus. The units in a column are also connected in common to the same source bus. Other electrically driven sensors, such as Hall-effect devices, can readily be added to the system and driven by the same matrix circuitry.
It has been empirically determined that the known system exhibits two anomalies. When incorporating the system in a high speed postage meter mailing machine, the sensors are principally used to monitor envelope flow through the mailing machine and determining physical properties of the envelope. As a result, sensor readings are taken at a very high rate. It has been observed that upon pulsing of he source line extraneous signals are initially produced which impacts both timing and system signal to noise ratio. In addition, due to the system timing requirements and the matrix arrangement, residue signals can be present on the data line further adversely effecting signal to noise ratios.